是否存在死循环

[flyonaie]

Describe the bug A clear and concise description of what the bug is.

template <typename T>
bool BoundedQueue<T>::Enqueue(const T& element) {
  uint64_t new_tail = 0;
  uint64_t old_commit = 0;
  uint64_t old_tail = tail_.load(std::memory_order_acquire);
  do {
    new_tail = old_tail + 1;
    if (GetIndex(new_tail) == GetIndex(head_.load(std::memory_order_acquire))) {
      return false;
    }
  } while (!tail_.compare_exchange_weak(old_tail, new_tail,
                                        std::memory_order_acq_rel,
                      

这段代码，old_tail值只是进入函数时更新了下，如果当前生产者没有获得可写入的索引，那下一次就可能无法退出循环，有可能等到整个下一次循环！

To Reproduce Steps to reproduce the behavior:

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2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior A clear and concise description of what you expected to happen.

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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context Add any other context about the problem here.

[MambaWong]

1. 当队列为满时，获取不到 index，则返回 false

    if (GetIndex(new_tail) == GetIndex(head_.load(std::memory_order_acquire))) {
      return false;
    }

2. 当没有其它线程同时入列时，则获取到 index，这时不用更新 old_tail，但需要更新 tail（即 new_tail）。
3. 当同时有其它线程入列且成功时（此时 tail 已在入列成功的线程中被更新），则先更新 old_tail（即 tail + 1），再重新获取 index，直到获取成功或队列满。